In general, the amplification of a modulation wave signal with a high peak-to-average power ratio causes a large distortion due to a poor linearity when back-off of an amplifier is made small for improvement of efficiency thereof. On the contrary, when the back-off is made large for improvement of the linearity, the efficiency is lowered. For example, a Doherty amplifier is provided as a method for improving the efficiency with the same back-off.
FIG. 9 shows a configuration of a conventional Doherty amplification circuit. The conventional Doherty amplification circuit includes a distributor 113 for dividing an input signal into two signals; a class AB or B carrier amplifier 111, a class C peak amplifier 112; a mixer 114 for combining an output signal from the carrier amplifier with an output signal from the peak amplifier; a characteristic impedance RL added to an output end of the carrier amplifier; a transmission line 116 having an electric length of θ+90°; a characteristic impedance RL added to an output end of the peak amplifier; a transmission line 118 having an electric length of Φ, and transmission lines 115 and 117 added to input ends of the carrier amplifier and the peak amplifier, respectively, to make a combined phase made by the mixer 114 in phase. In addition, a load impedance 103 of RL/2 is connected to an output end of the mixer.
During a low input power in which the peak amplifier 112 is not in operation, an output impedance Zd at the peak amplifier 112 usually becomes a complex impedance, not an open circuit impedance. Due to this, an output impedance Zd—p when viewing the peak amplifier from the mixer during the low input power is represented by using Zd, as follows:
                              Z                      d            -            p                          =                                                                              Z                  d                                ⁢                cos                ⁢                                                                  ⁢                Φ                            +                                                jR                  L                                ⁢                sin                ⁢                                                                  ⁢                Φ                                                                    cos                ⁢                                                                  ⁢                Φ                            +                              j                ⁢                                                      Z                    d                                                        R                    L                                                  ⁢                sin                ⁢                                                                  ⁢                Φ                                              .                                    Eq        .                                  ⁢                  (          1          )                    
The relationship between reflection coefficients is expressed by using Γd and Γd—p respectively normalized by RL, as follows:Γd-p=Γd·(cos 2Φ−j sin 2Φ)  Eq. (2).
FIG. 10 depicts a Smith chart representing the relationship between Γd and Γd—p in Eq. (2). The reflection coefficient Γd—p is shifted by an angle of 2Φ in a clockwise direction around RL. By changing the electric length Φ, the output impedance may be viewed as if it is an open circuit impedance.
Meanwhile, if a load impedance of the carrier amplifier is set to ZL(Low) at low input power and to ZL(High) at high input power, instead of representing a load impedance to be RL/2 at the low input power and RL at the high input power when viewing the mixer from the transmission line 118, ZL(Low) is represented with ZL(High) as follows:
                              Z                      L            ⁡                          (              Low              )                                      =                              Z                          L              ⁡                              (                High                )                                              ·                                                                      cos                  ⁢                                                                          ⁢                  θ                                +                                  j                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  sin                  ⁢                                                                          ⁢                  θ                                                                              2                  ⁢                                                                          ⁢                  cos                  ⁢                                                                          ⁢                  θ                                +                                  j                  ⁢                                                                          ⁢                  sin                  ⁢                                                                          ⁢                  θ                                                      .                                              Eq        .                                  ⁢                  (          3          )                    
A reflection coefficient r when viewing the load side from the carrier amplifier is represented as follows when normalized by ZL(High):
                    Γ        =                                            -              1                        3                    ⁢                                          ⁢                      (                                          cos                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                θ                            -                              j                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                θ                                      )                                              Eq        .                                  ⁢                  (          4          )                    
FIG. 11 is a Smith chart that represents the relationship between θ and Γ shown in Eq. (4). The reflection coefficient Γ is shifted by an angle of 2θ in a clockwise direction around ZL(High).
Under a large back-off condition, the efficiency of the Doherty amplifier improves by using ZL(High) and electric length θ, wherein ZL(High) set to a load impedance ZL(Pmax) that yields a maximum power and the transmission line 116 of θ+90° matches (transforms) the load impedance ZL(Low) to ZL(ηmax) performing maximum efficiency at low input power.
In this regard, an experimental result is reported on a Doherty power amplifier that employs an adaptive bias circuit applying an optimum gate bias according to the envelope of an input modulation wave signal (see, e.g., J. Cha, Y. Yang, B. Shin and B. Kim, “An Adaptive Bias Controlled Power Amplifier with a Load Modulated Combining Scheme for High Efficiency and Linearity”, 2003 IEEE MTT-S Digest, TU3B-4).
FIG. 3 is a graph showing one characteristic example of a gate bias voltage Vg which is supposed to be realized by the adaptive bias circuit. In the graph, the horizontal axis (algebra axis) represents an input power (instantaneous value) and the vertical axis indicates a gate bias voltage Vg.
In FIG. 3, a gate bias voltage A represents a gate bias voltage of a carrier amplifier; a gate bias voltage B indicates an optimum gate bias voltage of a peak amplifier when an adaptive bias is not applied thereto; an input power D denotes an input power yielding an average output power of a Doherty amplifier; and an input power E indicates a maximum input power by a peak-to-average power ratio of signal. Further, a region of oblique portion F represents the relationship between a gate bias voltage when the peak amplifier is not in operation and an input power.
The gate bias voltage shown in FIG. 3 is generated by detecting the envelope of an input signal and performing a linear operation of the detected envelope using an OP amp (operational amplifier), and is the function of an envelope power at that moment. It may be considered that the fundamental purpose of the peak amplifier is to adjust the input power D at which the peak amplifier starts to turn-on to the vicinity of the average power and then to amplify a peak exceeding an average power. However, the ratio of input powers D and E depends upon the configuration of the amplifier, which makes it almost impossible to change the ratio using the conventional Doherty amplifier starting at around 3 dB. (see, e.g., Japanese Patent No. 3372438).
Meanwhile, for example, in mobile communications that employs a multicarrier signal including a plurality of carrier waves, a peak-to-average power ratio is varied by the number of carrier waves. Further, an output power of a transmission amplifier for amplifying these signals is not also constant due to a transmission power control and so on. Accordingly, in case a linear amplification with high efficiency is performed in a wide dynamic range, it is required to apply an optimum bias dependent on a varying peak-to-average power ratio or an input power.
FIG. 12 shows a graph representing an adjacent channel leakage power ratio (ACLR) in a case where the gate bias of the peak amplifier is constant and in case of using the adaptive bias circuit as mentioned above in the conventional Doherty amplifier.
If a Doherty amplifier provided with the adaptor bias circuit is used in a certain power below an average input power, the gate bias of the peak amplifier is always set below the optimum gate bias B, thereby making it impossible to fully operate the peak amplifier when a signal having a high peak-to-average power ratio is inputted thereto. Therefore, the load modulation cannot be obtained, and thus, the load impedance at the carrier amplifier when the peak power is inputted thereto becomes almost ZL(Low) (ZL(η max)), which degrades a value of the ACLR at the time of a peak power exceeding an average output power G.
In addition, in the case of a signal having low peak-to-average power ratio, it is not possible to fully operate the peak amplifier. Therefore, a value of the ACLR is degraded at a certain power below the average output power.
As described above, in a Doherty amplifier, there is a need for improvement to apply an optimum bias dependent on a varying input power and/or a peak-to-average power ratio.